86 NITRIC ACID. 



pound already containing nitrogen is decomposed, we 

 find that, besides the compound of nitrogen and 

 hydrogen just described, a compound of oxygen and 

 nitrogen is also in some cases formed, which is pos- 

 sessed of very extraordinary powers, being a strong 

 acid, and commonly called aqua-fortis, or nitric 

 acid. 



164. Nitrogen is able to combine with oxygen in 

 five difi^erent proportions, giving rise to as many dis- 

 tinct substances ; four of these are of comparatively 

 little importance, and may therefore here be passed 

 over; the fifth, or that containing the greatest quan- 

 tity of oxygen, is the only one at present worthy of 

 consideration. 



165. Nitric acid is never found in its free or un- 

 combined state, but always in combination with some 

 base; being a very powerful acid, it is easy to under- 

 stand that, as soon as formed, it seizes upon some 

 base to combine with, and forms a neutral salt. The 

 commonest salts containing nitric acid, or nitrates, as 

 they are called, are the nitrates of potash, soda, and 

 lime, which are found native abundantly in difi*erent 

 places. 



166. The nitrate of lime is very frequently to be 

 met with on old walls, near which organic substances 

 containing nitrogen have decayed and formed nitric 

 acid, which has combined with the lime of the mortar; 

 and in the same way nitrate of lime may be formed 

 artificially, by mixing lime with decaying substances 

 which can yield the acid, and, after having allowed 



